The present invention relates to the repair of fiber optic cable systems and more particularly, the repair of an undersea cable system that uses dispersion slope-matched cable.
A fiber optic cable system is made up of cable containing optical fibers and repeaters containing optical amplifiers located periodically along the cable length. A fiber optic cable system can also include gain equalizers located periodically along the cable length. One important design parameter of such a system is chromatic dispersion, which relates to the velocity with which light at different wavelengths travels along the optical fibers. Dispersion as a function of system length needs to be managed if transmission performance of the system is to be optimized. One management method is to use sloped-matched cable, wherein the net end-to-end dispersion of each fiber path in the system is nominally constant across the transmission band and does not change with temperature.
In a typical Dense Wavelength Division Multiplex (DWDM) undersea slope-matched cable system, the xe2x80x9cregularxe2x80x9d or xe2x80x9ctransmissionxe2x80x9d cable sections (xe2x80x9ccable sectionxe2x80x9d refers to cable between adjacent repeaters) is made up of N-type and P-type fibers. These N-type and P-type fibers have large negative and positive dispersion rates (vs. distance), respectively, but in such proportion as to achieve a nominally constant net dispersion (e.g., a rate of about xe2x88x923 ps/nm-km2). The dispersion match preferably holds closely as a function of both wavelength and temperature.
Referring to FIG. 1, one fiber-pair makeup in a regular cable section 10 is shown with the thicker line representing the P-type fiber 12 and the thinner line representing the N-type fiber 14. Although one fiber pair is shown, an actual section typically contains multiple fiber pairs. In FIG. 1, optical signals are transmitted left to right in the upper fiber path and right to left in the lower fiber path. The slope-matched cable section 10 includes a middle portion 16 containing all-P-type fibers 12 and two end portions 18a, 18b containing both P-type and N-type fibers 12, 14. One example of the N-type fiber is available from Lucent under the designation IDFX2. One example of the P-type fiber is available from Lucent under the designation SLA. In the regular cable sections, the P-type fibers 12 and N-type fibers 14 are typically spliced together at the fiber factory using bridge fibers to minimize the net splice loss.
In addition to regular cable sections, compensation cable sections (not shown) containing all-P-type fibers can be used to manage the dispersion characteristic of the system appropriately. In one example of a slope-matched cable system, two or three xe2x80x9ccompensationxe2x80x9d cable sections in tandem are used every 450-500 km along the system length. FIG. 2 shows a dispersion map for an ideal double-compensating block length (using regular and compensation cable sections).
System gain equalization, separate from that which might be used in the repeater, in the form of, for example, Gain Equalizer Joints (GEJs), which correct for gain tilt, and Shape Compensating Units (SCUs), which correct for non-flat gain shape, can be placed in their own housings and be located in the compensation cable sections. To properly manage system dispersion, some compensation cable sections can be significantly shorter than regular cable sections and contain mid-section Loss Buildouts (LBOs). The LBOs are deliberately inserted amounts of optical attenuation in splice boxes to build out the loss to the cable section design value. A splice box is the main apparatus used to house fiber splices in a cable-to-cable joint.
Cable systems sometimes become faulted and must be repaired. Faults can occur in a portion of the cable or in a repeater connected to the cable. Referring to FIGS. 3-6, a repair operation to replace the faulted portion of an undersea cable 20 is described in greater detail. A typical undersea repair operation starts by cutting the cable 20 and retrieving what is expected to be the xe2x80x9cgoodxe2x80x9d end of the cable 20. Generally, the cable is engaged at a location B1 about a water depth D away from the cut so that there is equal weight on both sides during recovery (see FIG. 4). As a result, at least one water depth length D of the original cable section is removed. More cable (e.g., up to a kilometer) can be removed because of water ingress into the core cable structure from the cut end. After this cable is cleared, the end B1 of the cable 20 is sealed and it is buoyed off (see FIG. 5). The other cable end A2 is then retrieved and another water depth D or more length of the original cable 20 is removed. The fault is preferably in one of the lengths of cable 20 that was removed. If not, more cable is recovered until the cable is cut beyond the fault. If a faulty repeater is to be replaced, cable is recovered until it is onboard the ship, where it is cut out and replaced with a spare repeater.
Spare cable is then joined to the xe2x80x9cgoodxe2x80x9d end of the cable 20. The length of the spare cable is preferably enough to replace the original cable that was removed plus an additional length of typically 2 to 2.5 times the depth of water so that the final joint can be made before the cable bight is overboarded (see FIG. 6). For some existing broadband systems, it is prudent to insert an additional repeater, called a repair repeater, in the replacement cable to avoid a change in transmission gain shape (i.e., gain-tilt) that would occur because of the added loss resulting from the repair. Based upon the repair operation discussed above, the minimum amount of spare cable used during the repair (i.e., the replacement portion) is often 4.5 times the water depth at the repair site. The replacement portion might even be longer if more cable is removed due to water ingress into the otherwise good cable or not initially finding the fault in the recovered cable, or when a faulty repeater is retrieved.
Repairing slope-matched cable systems can present difficulties because the net end-to-end system dispersion following a repair should preferably remain unchanged. Even if the replacement section uses the same cable type that was removed, the added cable length will significantly unbalance the dispersion in both transmission directions.
Accordingly, a method of repairing a slope-matched cable system is needed that will allow the net end-to-end system dispersion following the repair to remain nominally unchanged in both transmission directions, even when a repair is made in a compensation cable section. A method of repairing a slope-matched cable system is also preferable such that it nominally leaves unchanged the gain tilt across the transmission spectrum.
To address the needs mentioned above, a slope-matched cable system is repaired using a replacement cable portion that leaves the net dispersion in the cable system nominally unchanged. In accordance with one aspect of the present invention, a method is used to repair a slope-matched cable system including at least one N-type fiber having a negative dispersion rate and at least one P-type fiber having a positive dispersion rate. The method comprises removing a faulted portion of the slope-matched cable system. First and second N-P cable lengths are provided from spare N-P cable including at least one N-type fiber and at least one P-type fiber. At least one all-P cable length is provided from spare P cable including at least first and second P-type fibers. A replacement cable portion is constructed from the N-P cable lengths and the all-P cable length, wherein the N-P cable lengths are connected to each side of the P cable length. During the repair, the replacement cable portion is connected between ends of the originally installed slope-matched cable where the faulted portion was removed. The method can also include connecting a repair repeater to one end of the replacement cable portion and/or connecting a replacement gain equalizer to the replacement cable portion when repairing a cable fault. The method includes connecting a replacement repeater to one end of the replacement cable portion and a repair repeater to the other end when repairing a repeater fault.
According to another aspect of the present invention, a replacement cable portion comprises first and second N-P cable lengths including half N-type fibers and half P-type fibers and at least one P cable length including all-P-type fibers. The N-type fibers have a negative dispersion rate and the P-type fibers have a positive dispersion rate. A first splice box connects the first N-P cable length and the P cable length. The N-type fibers in the first N-P cable length are spliced to a first half of the P-type fibers in the P cable length. The P-type fibers in the first N-P cable length are spliced to a second half of the P-type fibers in the P cable length. A second splice box connects the first N-P cable length and the P cable length. The N-type fibers in the first N-P cable length are spliced to the first half of the P-type fibers in the P cable length. The P-type fibers in the first N-P cable length are spliced to the second half of the P-type fibers in the P cable length. The replacement cable portion is preferably designed such that the net dispersion and gain shape of each fiber path in the system remains nominally unchanged when repaired with the replacement cable portion. Gain shape is preferably maintained by including, in the replacement portion, a repair repeater (and a replacement repeater when needed) with suitable Loss Buildouts (LBOs).